The invention relates to a heterodyne receiver with synchronous demodulation for receiving the radio signals from a time-signal transmitter, comprising an input circuit, a first mixer, driven by a first phase-locking loop that is controlled by a reference source, a bandpass filter and a second mixer for the direct mixing synchronous demodulator with output circuit connected behind in series, where the bandpass filter supplies an intermediate frequency to the second mixer.
A heterodyne receiver operating on the principle of synchronous demodulation is described in the publication DE 41 25 995 A1. In this receiver, a controlled oscillator arrangement is presynchronized in a presynchronization phase to a desired oscillator frequency through a first control loop with the help of a reference frequency source, generally the oscillator of an autonomous quartz clock with a frequency of 32768 Hz. This is followed by switching over to a second control loop and synchronizing to the received useful signal. In a synchronous demodulation, the frequency selection is performed by means of low-pass filters connected behind in series instead of a (quartz) bandpass filter. In accordance with DE 41 25 995 A1, the input signal is first converted to an intermediate frequency in order to avoid undesired interference of the antenna signal by the oscillator signal.
In order to bring the reception frequency of the receiver as accurately as possible to the desired value with the help of the reference frequency in the presynchronization phase, elaborate frequency dividers are necessary. Expense and space requirement in an integrated circuit and also the current requirement have a negative effect. This disadvantage is even greater if the receiver is to be designed for several reception frequencies because the expense of the control increases further. Owing to the relatively broad tolerances in the manufacture of integrated circuits, a wide interception range is necessary for presynchronization. On the other hand, owing to the signal-to-noise ratio, a much smaller interception range is required for the useful signal. This has the disadvantage, however, that if the transmitter fails for a short period of time or if there is a malfunction, the synchronization is usually lost and the ready-to-receive state must be established again by renewing the presynchronization.